Carbon fibrils are vermicular carbon deposits having diameters less than 500 nanometers. They exist in a variety of forms, and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces.
Tennent, U.S. Pat. No. 4,663,230, describes carbon fibrils that are free of a continuous thermal carbon overcoat and have multiple graphitic outer layers that are substantially parallel to the fibril axis. They generally have diameters no greater than 0.1 micron and length to diameter ratios of at least 5. Desirably they are substantially free of a continuous thermal carbon overcoat, i.e., pyrolytically deposited carbon resulting from thermal cracking of the gas feed used to prepare them.
Tubular fibrils having graphitic layers that are substantially parallel to the microfiber axis and having diameters broadly between 1.0 and 100 nanometers have been described in the art. Fibrils having diameters between 3.5 and 75 nanometers, are described in Tennent et al., U.S. Ser. No. 871,676 filed Jun. 6, 1986 (xe2x80x9cNovel Carbon Fibrils, Method for Producing Same and Compositions Containing Samexe2x80x9d), Tennent et al., U.S. Ser. No. 871,675 filed Jun. 6, 1986 (xe2x80x9cNovel Carbon Fibrils, Method for Producing Same and Encapsulating Catalystxe2x80x9d), Snyder et al., U.S. Ser. No. 149,573 filed Jan. 29, 1988 (xe2x80x9cCarbon Fibrilsxe2x80x9d), Mandeville et al., U.S. Ser. No. 285,817 filed Dec. 16, 1988 (xe2x80x9cFibrilsxe2x80x9d), Mandeville et al., U.S. Ser. No. 285,817 filed Dec. 16, 1988 (xe2x80x9cFibrilsxe2x80x9d), and McCarthy et al., U.S. Ser. No. 351,967 filed May 15, 1989 (xe2x80x9cSurface Treatment of Carbon Microfibersxe2x80x9d). Methods for manufacturing catalysts for producing carbon fibrils are described in Moy et al., U.S. Ser. No. 887,307, filed May 22, 1992 (xe2x80x9cImproved Methods and Catalysts for the Manufacture of Carbon Fibrilsxe2x80x9d). All of these patents and patent applications are assigned to the same assignee as the present application and are hereby incorporated by reference.
Fibrils are useful in a variety of applications. For example, they can be used as reinforcements in fiber-reinforced composite structures or hybrid composite structures (i.e. composites containing reinforcements such as continuous fibers in addition to fibrils). The composites may further contain fillers such as a carbon black and silica, alone or in combination with each other. Examples of reinforceable matrix materials include inorganic and organic polymers, ceramics (e.g., lead or copper). When the matrix is an organic polymer, it may be a thermoset resin such as epoxy, bisamaleimide, polyamide, or polyester resin; a thermoplastic resin; or a reaction injection molded resin. The fibrils can also be used to reinforce continuous fibers. Examples of continuous fibers that can be reinforced or included in hybrid composites are aramid, carbon, and glass fibers, alone, or in combination with each other. The continuous fibers can be woven, knit, crimped, or straight.
The composites can exist in many forms, including foams and films, and find application, e.g., as radiation absorbing materials (e.g., radar or visible radiation), adhesives, or as friction materials for clutches or brakes. Particularly preferred are fibril-reinforced composites in which the matrix is an elastomer, e.g., styrene-butadiene rubber, cis-1,4-polybutadiene, or natural rubber.
In addition to reinforcements, fibrils may be combined with a matrix to create composites having enhanced thermal, and/or electrical conductivity, and/or optical properties. They can be used to increased the surface area of a double layer capacitor plate or electrode. They can also be formed into a mat (e.g., a paper or bonded non woven fabric) and used as a filter, insulation (e.g., for absorbing heat or sound), reinforcement, or adhered to the surface of carbon black to form xe2x80x9cfuzzyxe2x80x9d carbon black. Moreover, the fibrils can be used as an adsorbent, e.g., for chromatographic separations.
Fibrils are advantageously prepared by contacting a carbon-containing gas with a metal catalyst in a reactor at temperature and other conditions sufficient to produce them with the above-described morphology. Reaction temperatures are 400-850xc2x0 C., more preferably 600-700xc2x0 C. Fibrils are preferably prepared continuously by bringing the reactor to the reaction temperature, adding metal catalyst particles, and then continuously contacting the catalyst with the carbon-containing gas.
Examples of suitable feed gases include aliphatic hydrocarbons, e.g., ethylene, propylene, propane, and methane; carbon monoxide; aromatic hydrocarbons, e.g., benzene, naphthalene, and toluene; and oxygenated hydrocarbons.
Preferred catalysts contain iron and, preferably, at least one element chosen from Group V (e.g., vanadium), Group VI (e.g. molybdenum, tungsten, or chromium), Group VII (e.g., manganese), Group VIII (e.g. cobalt) or the lanthanides (e.g., cerium). The catalyst, which is preferably in the form of metal particles, may be deposited on a support, e.g., alumina and magnesia.
The carbon fibrils produced by these catalysts have a length-to-diameter ratio of at least 5, and more preferably at least 100. Even more preferred are fibrils whose length-to-diameter ratio is at least 1000. The wall thickness of the fibrils is about 0.1 to 0.4 times the fibril external diameter.
The external diameter of the fibrils is broadly between 1.0 and 100 nanometers and preferably is between 3.5 and 75 nanometers. Preferably a large proportion have diameters falling within this range. In applications where high strength fibrils are needed (e.g., where the fibrils are used as reinforcements), the external fibril diameter is preferably constant over its length.
Fibrils may be prepared as aggregates having various macroscopic morphologies (as determined by scanning electron microscopy) in which they are randomly entangled with each other to form entangled balls of fibrils; or as aggregates consisting of bundles of straight to slightly bent or kinked carbon fibrils having substantially the same relative orientation in which the longitudinal axis of each fibril (despite individual bends or kinks) extends in the same direction as that of the surrounding fibrils in the bundles; or, as aggregates consisting of straight to slightly bent or kinked fibrils which are loosely entangled with each other to form a more open structure. In the open structures the degree of fibril entanglement is greater than observed in the parallel bundle aggregates (in which the individual fibrils have substantially the same relative orientation) but less than that of random entangled aggregates. All of the aggregates are dispersable in other media, making them useful in composite fabrication where uniform properties throughout the structure are desired. In the parallel bundle aggregates the substantial linearity of the individual fibril strands, which are also electrically conductive, makes the aggregates useful in EMI shielding and electrical applications.
The macroscopic morphology of the aggregate is influenced by the choice of catalyst support. Spherical supports grow fibrils in all directions leading to the formation of random, entangled aggregates. Parallel bundle aggregates and aggregates having more open structures are prepared using supports having one or more readily cleavable planar surfaces, e.g., an iron or iron-containing metal catalyst particle deposited on a support material having one or more readily cleavable surfaces and a surface area of at least 1 square meters per gram.
Preferred support materials include the various aluminas (stoichiometries corresponding to Al2O3.H2O or AlO.OH), or gamma-alumina (Al2O3) or magnesia (MgO). Additionally, hydrous aluminas (stoichiometries corresponding to Al(OH)3 or Al2O3.3H2O), calcined lightly at temperatures below about 800xc2x0 C. yield activated aluminas (Al2O3.H2O) which retain the platelet morphology of the initial hydrous alumina. These result in highly preferred supports. Such materials are commercially available, e.g., from ALCOA (hydrous and activated aluminas) and Martin Marietta (magnesia). The activated alumina supports yield primarily parallel bundle aggregates, while the magnesia supports yield primarily the more open aggregates. Spherical gamma alumina particles, which yield random entangled aggregates, are available from Degussa.
It is believed that deposition of fibril growth catalysts on supports having planar surfaces allow the fibrils to orient themselves with each other as they grow, creating a xe2x80x9cneighbor effectxe2x80x9d. This leads then to a parallel bundle fibril aggregate in which the areas of all of the individual fibrils have the same relative orientation. The magnesia supports, although having readily cleavable planar surfaces, yield primarily lightly entangled, open net fibril aggregates because they break apart more readily into smaller particles than the activated alumina support during fibril growth, resulting in aggregates that are less ordered than the parallel bundle aggregates but more ordered than the tightly entangled fibril balls. The more readily the oxide and support can form a mixed oxide at the interface between them, the more likely the support is to break apart.
Further details regarding the formation of carbon fibril aggregates may be found in the disclosure of Snyder et al., U.S. patent application Ser. No. 149,573, filed Jan. 28, 1988, and PCT Application No. US 89/00322, filed Jan. 28, 1989 (xe2x80x9cCarbon Fibrilsxe2x80x9d) WO 89/07163, and Moy et al., U.S. patent application Ser. No. 413,837 filed Sep. 28, 1989, PCT Application No. US 90/05498, filed Sep. 27, 1990 (xe2x80x9cFibril Aggregates and Method of Making Samexe2x80x9d) WO 91/05089, and Tennent, et al, U.S. patent application Ser. No. 057,328, filed May 5, 1993 (xe2x80x9cThree Dimensional Macroscopic Assemblages of Randomly Oriented Carbon Fibrils and Composites Containing Samexe2x80x9d), all of which are assigned to the same assignee as the invention here and are hereby incorporated by reference.
Fibrils are increasingly important in a variety of industrial uses. While known methods of manufacture permit production of small quantities of fibrils, it is important to improve these methods, and in particular the catalysts used in those methods, to increase the yield of fibrils, to improve their quality and to lower their cost of production. It is also desirable to produce carbon fibrils of improved purity.
Furthermore, it is desirable to produce fibrils with enhanced dispersability and electrical conductivity properties. It is important to improve the ability of fibrils to disperse in media. In particular, it is desirable to increase the ability of fibrils to disperse in thermoplastics or engineering plastics. Dispersion of fibrils into media also imparts enhanced electrical conductivity properties to said media.
It is thus a primary object of the invention to provide improved catalysts for the production of fibrils.
It is a further object of the invention to provide catalysts for the production of fibrils that are more readily dispersed in media.
Another object of this invention is to provide catalysts yielding fibrils capable of imparting enhanced electrical conductivity properties to a media.
It is also an object of the invention to increase the yield and productivity of fibril-producing catalysts.
It is still a further object of the invention to provide improved methods of preparing fibril-producing catalysts.
It is yet another object of this invention to improve the quality and uniformity of fibrils and aggregates thereof.
It is still a further related object of the invention to improve the economics and reliability of fibril manufacture.
Methods have now been found which yield catalysts that produce substantially superior carbon fibrils and carbon fibril aggregates. These catalysts can be obtained by contacting a fibril-forming catalyst or precursors of a fibril-forming catalyst with an effective amount of a surfactant and/or polyol. The method is preferably carried out by precipitating a fibril-producing metal oxide or compound from an aqueous solution onto slurried particles of a support material in the presence of surfactant and/or polyol.
This invention further provides a catalyst for the production of carbon fibril aggregates produced by the method of contacting a fibril-forming catalyst or precursors of a fibril-forming catalyst with an effective amount of a surfactant and/or polyol. Preferably, the catalyst is formed by precipitating a fibril-producing metal oxide or compound from an aqueous solution onto slurried particles of a support material in the presence of a surfactant and/or polyol.
Also provided by this invention is a volume of carbon fibrils comprising a multiplicity of fibrils that are free of a continuous thermal carbon overcoat, have graphitic layers that are substantially parallel to the fibril axis, and possess a substantially constant diameter. In a preferred embodiment the diameter of the fibrils is from about 4.0 to about 20 nanometers.
The improved methods of making fibril-forming catalysts and the improved catalysts themselves produce superior carbon fibrils and carbon fibril aggregates possessing enhanced dispersion and electroconductivity qualities. The resultant carbon fibrils exhibit improved characteristics that enable fibrils or fibril aggregates to disperse better in a media. Additionally, the carbon fibrils produced by the improved catalysts provided by this invention impart increased electroconductivity to the media in which they are dispersed.
The term xe2x80x9cfibril-forming catalystxe2x80x9d is used to refer collectively to catalysts for forming discrete carbon fibrils, carbon fibril aggregates or both.
The term xe2x80x9ccarbon fibrilsxe2x80x9d when referring to products is used to refer collectively to both discrete carbon fibrils and carbon fibril aggregates, unless the context indicates a different meaning.
This invention provides a method for the manufacture of a catalyst for the production of carbon fibrils comprising contacting a fibril-forming catalyst or precursors of a fibril-forming catalyst with an effective amount of a surfactant and/or polyol. The method for the manufacture of a catalyst for the production of carbon fibrils preferably comprises the steps of forming an aqueous solution of a Period Four transition metal iron compound or a Period Four transition metal and molybdenum compound, forming a slurry of catalyst support particles comprising alumina and/or magnesia particles, precipitating an iron compound or iron and molybdenum compounds onto the alumina and/or magnesia particles in the presence of a surfactant and/or polyol, and then processing the slurry to produce a fibril-forming catalyst.
Preferably, the surfactant is stable at pH levels from about 3 to about 9 and does not itself cause precipitation of ferric oxide or compounds. Members from the usual classes of anionic, cationic or non-ionic surfactants are effective. In one embodiment of the invention the surfactant is non-ionic. The preferred non-ionic surfactants include ethoxylated alkyl phenols, other ethoxylated and/or propoxylated derivatives, and functionalized organosiloxanes.
In another embodiment of this invention, the surfactant is an anti-foaming agent. Examples of anti-foaming agents include substituted nonylphenols, organo-modified polysiloxanes, and emulsified silicone formulations.
In other preferred embodiments of the invention the surfactant can be ethylene oxide-propylene oxide copolymers, substituted alkyl phenols, alkali metal salts of polymeric carboxylic acids, derivatized polyalkylsiloxanes, ethoxylated amines, quaternary amine salts and derivatized nitrogen compounds (such as imidazoles and pyrimidines).
Examples of preferred polyols used in certain embodiments of this invention include glycerine, sucrose and polyethylene glycol.
A preferred method of manufacturing a catalyst for the production of carbon fibrils comprises forming an iron or iron and molybdenum salt solution, forming a slurry of catalyst support particles comprising alumina particles, precipitating iron or iron and molybdenum oxide onto said alumina particles in the presence of a surfactant, anti-foam agent or polyol at a pH of about 6, then filtering and washing the slurry followed by drying at about 140xc2x0 C. to about 200xc2x0 C. yield a fibril-forming catalyst.
Embodiments of the invention include, but are not limited to, adding soluble surfactant and/or anti-foam and/or polyol to the iron or iron and molybdenum aqueous solution; adding surfactant and/or anti-foam and/or polyol to the alumina or magnesia slurry; and adding surfactant and/or anti-foam and/or polyol to both the iron or iron/molybdenum aqueous solution and the slurry of alumina or magnesia.
This invention also provides a catalyst for the production of carbon fibrils that are produced by contacting a fibril-forming catalyst or precursors of a fibril-forming catalyst with an effective amount of a surfactant and/or polyol.
Further provided by this invention is a volume of carbon fibrils comprising a multiplicity of fibrils having a morphology consisting of tubes that are free of a continuous thermal carbon overcoat, graphitic layers that are substantially parallel to the fibril axis, and a substantially constant diameter. Preferably, the diameter of the fibrils is from about 4.0 to about 20 nanometers.
The immediate improvement in the catalysts of this invention are seen in the fibrils which they produce. They make more uniform fibrils of smaller diameter (from about 4.0 to about 20 nanometers most preferably 7-10 namometers) thereby increasing the surface area of fibrils. In addition, the aggregates, which resemble parallel bundle aggregates, are much smaller (approximately 0.1-1 micron, and some as small as about 0.1 micron). This results in fibril aggregates which are much easier to disperse and thereby impart higher electrical conductivity to the dispersed medium. The smaller aggregates also allow for dispersions of fibrils to nearly the individualized state (absence of bundles or other fibril aggregates) leading to open, three-dimensional network mats.
While not wishing to be bound by any theory, it is believed that the improved properties of the fibrils (i.e., improved dispersibilities in thermoplastics and/or polymers, and improved electrical conductivities in these formulations, and the ability to make open, three-dimensional network mats from superior dispersions) results from better dispersion of iron or iron/molybdenum oxide particles which are deposited on the surface of the support in the presence of surfactant, anti-foam agent or polyol. The surfactant, anti-foam or polyol interacts with the surface of the precipitated iron or iron/molybdenum oxide to decrease particle-particle interaction, stabilizing the small aggregate particles by retarding the growth or sintering into larger aggregates. The smaller iron or iron/molybdenum particles also lead to fibrils with smaller average diameters.
One class of surfactants used (although not limited to) are non-ionic, particularly alkylated phenols, ethoxylated alkyl phenols, alkoxylated derivatives and functionalized organosiloxanes. Examples of commercial surfactants which may be classified more narrowly as xe2x80x9cdispersantsxe2x80x9d or xe2x80x9canti-foam agentsxe2x80x9d are Triton X-100 (ethoxylated nonylphenol, Rohm and Haas) or Anti-Foam A (Organomodified polysiloxane, Sigma).
Again, while not wishing to be bound by any particular theory, it is believed that a second pathway by which these catalysts yield improved fibril aggregates is by facilitating the breaking apart of the support (activated alumina or magnesia) particles. The preferred support for these catalysts are flat, planar hydrous alumina (Al(OH)3) platelets which have been lightly calcined from about 225 to about 800xc2x0 C. to a composition approaching activated alumina, Al2O3.H2O, without any substantial change in the platelet structure. The weight loss on calcination is 27-33 wt % H2O.
The aggregate particles of the support are made up of submicron, flat platelets which are loosely held together into aggregates by binding through surface hydroxyl ions. Iron, or iron/molybdenum oxide particles are deposited on the surface of and the crevices between platelets. These platelets then separate and break apart into smaller platelets from the heat of reaction and the force of the fibrils. The planar structure of the support then orients the individual growing fibrils into a CY macromorphology.
The use of surfactants or polyols are believed to decrease the inter-platelet or inter-particle attractions by exchanging, neutralizing or binding surface hydroxy groups which then allows the plates to be cleaved more easily yielding smaller plates and thereby smaller fibril bundles (sub-micron in size). These smaller bundles (0.1-1 micron) are then easier to disperse than larger bundles (0.5-2 micron) obtained with conventional catalysts.
Again, while not wishing to be bound by any particular theory, it is believed that a second pathway by which these catalysts give improved performance is by facilitating the breaking apart of alumina particles. The preferred slurry support for catalysts are hydrous aluminas (Al(OH)3) which have been lightly calcined to greater than about 27% weight loss.
The aggregate particles of the support are made up of submicron, flat plates which are held together by binding through surface hydroxy ions. Iron oxide particles are deposited on the open surfaces and in the crevices between platelets. As fibrils undergo growth the planar surfaces orient the individual fibrils in the parallel bundle morphology.
The resulting bundles are easier to disperse than conventional larger bundles because the sub-micron dimensions of the plates produce very small bundles (diameters as small as about 0.1 micron). Additionally, the use of surfactants to decrease the inter-particle attractions by exchanging, neutralizing or binding surface hydroxy groups, allows the plates to be cleaved more readily, yielding smaller plates and smaller bundles.
Examples of surfactants, which may also be more narrowly defined as Dispersants or Anti-Foams are Triton X-100 or Tamol-731 (Rohm and Haas), EPO-61 (ethylene oxide-propylene oxide co-polymer from Harcros), HL-36 or Anto-Foam 204 (non-silicone Anti-Foams available from Harcros and Sigma, respectively).
Other preparations combine a surfactant, anti-foam agent, or polyol added through the iron/molybdenum solution and a surfactant added to the slurry of alumina support. The precipitation is carried out as described below. These preparations use other types of surfactants besides antifoams. Other surfactants were formulations of ethylene oxide-propylene oxide co-polymers, substituted alkylphenols, or alkali metal salts of polymeric carboxylic acids. Other surfactant formulations also include derivatized polyalkylsiloxanes, ethoxylated amines, quaternary amine salts, derivatized nitrogen compounds (e.g. imidazoles, pyrimidines) or any of the class of surfactants (cationic, anionic or non-ionic) which are stable at pH levels from about 3 to about 9 and by themselves do not cause precipitation of ferric ions.
Method
All the catalysts were prepared by precipitation of iron and molybdenum oxides at a controlled pH. The surfactant or polyol could be added to the Fe/Mo salt solution from which the oxides were precipitated, to the alumina slurry, or both. The support for all catalyst examples was a hydrous alumina (Al(OH)3 or Al2O3.3H2O) available from Alcoa, designated H-705, which had been lightly calcined between 280-600xc2x0 C. to give about 27-33 percent weight loss to give an activated alumina with composition Al2O3.H2O).